Conventionally, a material which uses asbestos as a main friction material is generally known for use in brakes for trucks, automobiles and the like. However, there is a demand for friction materials not using asbestos from the viewpoint of safety and hygiene Japanese Patent Laying-Open No. 54-34349, Japanese Patent Publication No. 64-4541, Japanese Patent Publication No. 57-37187 and Japanese Patent Publication No. 59-4459 disclose conventionally and generally known friction materials not using asbestos. Japanese Patent Laying-Open No. 54-34349, which corresponds to U.S. Pat. Nos. 4,273,699 (Chester) and 4,310,452 (Chester) discloses a friction material obtained by forming a mixture of phenol-formaldehyde resin and fibers such as steel fibers or glass fibers and an inorganic reinforcement material other than asbestos. Mica is listed as one of the grain reinforcing materials.
Japanese Patent Publication No. 64-4541 discloses the use of mica as a filler in a friction material using glass fibers and fused liquid crystalline polyester as a matrix material of a polymer binder in order to change the properties of the friction material and to reduce manufacturing costs.
Japanese Patent Publication No. 57-37187 discloses a friction material including a fibrous substance of 0-5 vol. %, scaly or layered mica of 20-60 vol. %, phenol resin of 5-30 vol. %, BaSO.sub.4 of 5-30 vol. %, cashew dust of 5-15 vol. % and metal particles of 5-30 vol. %. It is mentioned that mica can replace asbestos as a main friction material.
Japanese Patent Publication No. 59-4459 teaches mixing coke powder and cashew dust as a friction material, whereby the coke powder becomes enveloped with cashew dust or the coke powder is diffused in the cashew dust.
The foregoing shows that a number of friction materials not using asbestos have been conventionally proposed. However, no known material has been developed that costs the same as an asbestos containing material and has the characteristics of asbestos such as flexibility, strength, wearing resistance and softness to provide a substitute for asbestos and consequently a plurality of different fibers are used together to provide a substitute for asbestos. However, most of the substitute fibers are thick and stiff and have such characteristics that separation and segregation between fibers and other powder material might occur dependent on the treatment after mixing mica and other powder materials. Separation and segregation prevent the formation of a homogeneous mixture.
In addition, since the powdery and scaly form of mica which can be used effectively as a substitute for asbestos, can explode, it is very difficult to form a homogeneous mixture. It is also difficult to mix mica and the like with resin or the like used as a binder or matrix, whereby the bonding between the mica and the binder may be insufficient, so that the friction material has a lower strength or cracks may be caused by expansions due to gas occurring during the formation and curing. Using an increased amount of resin as the binder for avoiding these problems, allows the heat resistance of the friction material to deteriorate, thereby furthering the expansion due to gas formation. Hence, it is difficult to manufacture a mica containing friction material of uniform quality as an effective substitute for a friction material containing asbestos.
Another conventional problem to be solved with respect to the characteristics required of a friction material for brakes, is brake noise occurring at the time of braking. It is difficult to effectively prevent such brake noise. Generally, a friction material containing a large amount of solid lubricant such as graphite is well-known as a friction material which is unlikely to cause brake noise. However, if a large quantity of graphite or the like is used, the friction coefficient .mu. is decreased resulting in a lower braking force. Also effective for preventing brake noise in braking, is a friction material containing a large quantity of organic filling materials such as rubber and cashew dust, as is disclosed in, for example, Japanese Patent Publication No. 59-4459 as mentioned above. However, such friction material also has a decreased friction coefficient .mu. when a brake is applied at a high speed. Such brake materials also generate more than normal heat, thereby increasing wear and tear.
U.S. Pat. No. 4,735,975 (Iwata et al.) discloses a brake pad material in which the granular structure is rigid as will be described in more detail below with reference to FIG. 12. Iwata et al. require this rigidness in the granular structure to avoid deformation or disturbance of the granular structure in the final thermal and pressure deformation applied for forming the brake pad material. Such a rigid granular structure cannot reduce brake noise because the particles within the granular structure are not easily displaced relative to each other when a brake force is applied.